1. Field of the Invention
This invention relates to an information recording/reproduction apparatus for recording and reproducing information in a high density by utilizing a near field light emission device having a minute aperture for generating near field light, as a near field optical head.
2. Description of the Related Art
A near field light emission device has been utilized or examined at present as a near field optical head of an information recording/reproduction apparatus and as a probe for optically observing a sample.
Large capacity and small scale have been requisite for information recording/reproduction apparatuses using light and to achieve this object, higher density of a recording capacity has been necessary. Though research using a blue-violet semiconductor laser have been conducted, these technologies can improve only several times the recording density of the present level due to the problem of the diffraction limit of light. In contrast, an information recording/reproduction method utilizing near field light would be a promising method as a technology that handles optical information of a minute region exceeding the diffraction limit of the light.
This technology utilizes near field light generated in the proximity of an optical aperture having a size below the wavelength of light and formed in a near field optical head as a near field light emission device. In this way, the technology can handle optical information in a region below the wavelength of light which region is believed to be the limit in the conventional optical systems. Reproduction methods of the optical information include a method (collection mode method) that irradiates light to the surface of a recording medium and converts near field light locally existing at a minute mark to scattered light by the interaction with the minute aperture, and a method (illumination mode method) that irradiates near field light generated from the minute aperture to the surface of the recording medium and detects scattered light converted by the interaction with the surface of the recording medium the optical constants of which change, such as minute concavo-convexities on which the information is recorded, the refractive index, etc., by use of a light reception device disposed separately. Recording is made by a method (heat mode recording) that irradiates near field light generated from the minute aperture to the recording medium surface and changes the shape of the minute region on the medium, and a method (photon mode recording) that changes the refractive index or transmissivity of the minute region. Higher density than that of the conventional information recording/reproduction apparatuses can be achieved by use of the near field optical head having the minute optical aperture exceeding the diffraction limit of light.
The construction of the recording/reproduction apparatus using near field light is generally and substantially the same as that of magnetic disk apparatuses, and the apparatus uses the near field optical head in place of a magnetic head. The near field optical head having the minute optical aperture fitted to the distal end of a suspension arm is caused to float to a predetermined height in accordance with a flying head technology, and access is made to an arbitrary data mark existing on the disk. To let the near field optical head follow the disk rotating at a high speed, a flexure function is provided that stabilizes the posture of the head in such a fashion as to respond to swell of the disk.
A method of supplying light to the near field optical head having such a construction comprises the steps of connecting an optical fiber or an optical waveguide to the near field optical head, and irradiating a luminous flux from a laser as a light source to the minute aperture formed in the near field optical head.
In the information recording/reproduction apparatus described above, the luminous flux irradiated from an end face of the optical waveguide and having an expansion angle is reflected by a mirror, or the like, and is then irradiated to the minute aperture. Therefore, the energy density of light becomes low in the proximity of the minute aperture with the result that the intensity of near field light occurring in the proximity of the minute aperture becomes low, too.
Therefore, a lens is interposed between the end face of the optical waveguide and the minute apparatus so as to condense the luminous flux irradiated from the end face of the optical waveguide to a portion in the proximity of the minute aperture, to increase the intensity of near field light occurring in the proximity of the minute aperture and to improve light utilization efficiency. When a lens having a high NA is used, a condensation spot size can be made small and optical energy can be concentrated on a finer region. When the minute aperture is disposed at this condensation point, the intensity of near field light occurring in the proximity of the minute aperture can be increased and the luminous flux from the laser can be utilized efficiently.
In such an information recording/reproduction apparatus, however, the number of components such as optical waveguides, mirrors, and the like, increases. Since the number of positions to be adjusted increases, too, the drop of performance and the increase of the adjustment time as well as the production cost will occur.
In Japanese Patent Laid-Open No. 2000-215494, Ohkubo et al. provide an optical information recording/reproduction apparatus having the following extremely simple construction. The apparatus uses a substantially rod-like optical waveguide having flexibility. A reflection surface is disposed on the side of one of the ends of the optical waveguide to reflect at least a part of light propagating inside a core to a clad-transmitting direction. A light shading film for cutting off transmission of light is formed on the surface of a clad with the light transmission portion of light reflected on the reflection surface as a center. An aperture smaller than the wavelength of light to be used is formed by removing a part of the light shading film corresponding to the light transmission portion of light reflected by the reflection surface. In this way, a cantilever type optical pickup capable of generating near field light from the lower surface of the tip is accomplished.
The information recording/reproduction apparatus produced by discretely preparing and assembling the optical waveguide, the mirror, the lens, and the like, can generate sufficiently strong near field light by use of the minute aperture, and can accomplish recording and reproduction of ultra-high density information and a high signal-to-noise ratio (SN). However, because the optical guide (thin film optical waveguide, optical fiber, etc.), the mirror and the lens are necessary to efficiently guide the luminous flux from the light source to the minute aperture of the near field optical head having the minute aperture, the number of components increases. The increase of the number of the components results in the increase of the number of adjustment positions and in the increase of the production cost. Since the components thus increased invite the increase of the mass of the near field optical head, high-speed tracking becomes more difficult, and high-speed recording/reproduction of information becomes more difficult, too.
In the information recording/reproduction apparatus using the optical pickup of Japanese Patent Laid-Open No. 2000-215494 by Ohkubo et al, the luminous flux propagating inside the core of the optical waveguide is guided to the minute aperture by the reflection surface that reflects light in the clad-transmitting direction. However, because the luminous flux reflected from the core having the reflection surface towards the clad is guided to the minute aperture while it is being diverged with an expansion angle, the energy density of the luminous flux drops at the minute aperture, and sufficiently strong near field light cannot be generated in the proximity of the minute aperture. To highly improve the NA of the luminous flux incident into the minute aperture and to increase the energy density of the luminous flux at the minute aperture, a minute ball lens is interposed between the minute aperture and the core having the reflection surface formed thereon so that the energy density of the luminous flux guided to the minute aperture can be increased.
A high NA of the luminous flux incident into the minute aperture can be achieved by increasing the diameter of the luminous flux incident into the lens and by using a lens having a small focal distance. To increase the diameter of the luminous flux incident into the lens, it is necessary to secure a sufficient distance from the core end face to the lens.
The substantially rod-like optical guide having flexibility generally has a thickness of not greater than about 100 μm, and it is difficult to attain a high NA for increasing the energy density of the luminous flux incident into the minute aperture. It would be possible to attain a high NA of the luminous flux incident into the minute aperture by increasing the distance between the core having the reflection surface formed thereon and the ball lens, but when such an arrangement is employed, the optical waveguide loses its flexibility. In addition, since the optical pickup becomes thicker and the position of the center of gravity becomes higher, high-speed tracking becomes difficult to execute.